In metal electrowinning a current is passed from an inert anode to a cathode through a liquid leach solution containing said metal so that the metal is extracted as it is deposited onto the cathode. A significant part of the specific electrical energy consumption (SEEC) for this process is due to the reaction which occurs at the anode. In the case of copper this represents over 25% of the total energy requirement of the copper production.
In sulfate based electrolytes the anode reaction is oxygen evolution, caused by electrolytic splitting of water into protons and oxygen. This provides electrons for the reduction of metal cations at the cathode. Sulfate based electrolytes are used, for instance, in electrowinning of copper, zinc, nickel, chromium and manganese.
In metal electrowinning from sulfate (or sulfuric acid) based electrolytes, the oxygen evolution reaction that occurs at the anode is given by the following equation:H2O→2H+(aq)+½O2(g)+2e−  (1)E0=+1.23 V vs. SHE
The overall reaction for copper electrowinning with an oxygen-evolving anode is given by equation (2). The reaction produces one mole of cathode copper, one mole of sulfuric acid and half a mole of oxygen gas:CuSO4(aq)+H2O→Cu(s)+H2SO4(aq)+½O2(g)  (2)Ecell=+1.7 to 2.0 V vs. SHE
Efficiency and cost-effectiveness of electrowinning is important for the competitiveness of metal industry. The electrical energy cost of metal electrowinning is almost directly proportional to cell voltage.
Attempts have been made to develop anodes that would reduce the energy required for electrowinning. These attempts comprise, for instance, modification of lead anodes and switching to dimensionally stable anodes (DSA). In most cases, the anticipated energy savings have been in the region of a few hundreds of millivolts, or 5-15% of the cell voltage.
Dimensionally stable anodes comprise a thin active coating, usually few microns, deposited on a base metal, such as Ti, Zr, Ta, Nb. The coating enables the electrical charge transport between the base metal and the electrode/electrolyte interface, and is chosen for its high chemical and electrochemical stability and its ability to catalyze the desired electrochemical reaction.